The seminars listed here can be given upon request. A biographical sketch can be found
at the bottom of this page.
Workshop: The Art of Science
The current model for training researchers is
very much like the medieval system where an apprentice follows a
master for years of training. This model gives graduate students
valuable hands-on experience. What often lacks in this
educational model is an explicit transfer of skills and
information at a rate and moment in time that is effective for
acquiring research skills in a timely manner. For this reason I
started the course "The Art of Science" at the Colorado School
of Mines. I teach this material in different forms; as a
semester-long course, as a short course of a few afternoons, or
as a single one-hour seminar. Depending on the length of the
course and the wishes of the audience I choose from the
Tutorial: seismic interferometry, who needs a
seismic source? (download ppt)
Seismic interferometry is a technique for imaging
without coherent sources. The idea is to combine waveforms,
generated by ambient noise, that are recorded at different
receivers in a way to provide the waves that would propagate
between these receivers as if there was a source at one of these
receivers. This obviates the need to have a soure located at one
of the receivers. In the tutorial I cover different formulations
of the theory that explain seismic interferometry, and present
examples with field data that show the possibilities that are
opened up with this new technique. With the advent of permanent
networks of seismometers in exploration seismology and global
seismology, seismic interferometry opens up new methods for
imaging and monitoring.
Title: Extraction of the Green's function from
ambient fluctuations for general linear systems
Title: Extracting the building response from
Structures such as buildings or bridges are often
instrumented with acellerometers to monitor the vibrations.
Since the excitation of these structures usually is incoherent,
these recordings do not directly give the impulse response (the
response to an impulsive loading) of these structures. I show
how seismic interferometry can be used to extract the impulse
response from a building from incoherent vibrations recorded in
a building after an earthquake. I also show that depending on
the data-processing that is applied, either the propagating
waves or the normal modes of the buliding can be retrieved. With
this apprach the response of the building can be separated from
the coupling of the building to the subsurface. In this seminar
I show the theory and apply this to the motion recorded in the
Millikan Libary in Pasadena (California).
Multiple scattered waves are not very useful for deterministic imaging in complicated media because there is no known algorithm to construct such an image. Because multiple scattered waves have long wave-paths, these waves are very sensitive to small changes in the medium. Coda wave interferometry is a new technique that can be used to detect minute changes in a strongly scattering medium using changes in the multiple scattered waves over time. This technique is analogous to speckle pattern interferometry as used in optics, but takes advantage of the phase information in recorded waves. Because of its modest hardware requirements, coda wave interometry has a large number of applications. These include geotechnical applications (dam-monitoring, tunnel monitoring), the evaluation of hazards (volcano and fault monitoring), non-destructive testing, locating earthquakes, and monitoring of hydrocarbon reservoirs.
Title: Time-reversed imaging as a diagnostic of wave and particle chaos
Chaotic behaviour of particles concerns the stability properties of trajectories under perturbations of initial conditions. For waves, chaotic behaviour is less clearly defined. Both Newton's law and the Helmholtz equation are symmetric under time-reversal. This means that particles or waves emitted by a source at t=0 should refocus on the source when their propagation is reversed in time. Chaotic behaviour will prevent this to occur. This idea is tested for a system of very strong scatterers through which particles and wave propagate. Analytical expressions are derived for the critical perturbations of the initial conditions of both waves and particles. It is shown that the resulting behaviour of waves and particles are fundamentally different with critical length scales ranging over 15 orders of magnitude. The analytical results are illustrated and confirmed by numerical simulations.
Title: The arrow of time
It is great irony of science that the most
fundamental concepts are often most dificult to understand. The
concept "time" is an important example of this. The laws that
describe the basic forces in nature are symmetric for time
reversal. This means that they do not change when one changes
the direction of time by replacing the time t by -t. However,
this clearly contradicts our experience; we perceive a direction
of time. This direction is called the "arrow of time." Different
arrows of time can be distinguished: the thermodynamic arrow of
time, the biological arrow of time, the radiative arrow of time,
the mechanical arrow of time and the cosmological arrow of time.
The relation with natural laws that are not invariant for time
reversal is discussed and some pitfalls are shown in "deriving"
equations with a direction of time from the fundamental laws of
physics. The symmetry of time reversal has important
applications in geophysics, examples are shown of this. The
final question remains: "what explains the arrow of time that
seems to pervade our daily experience?"
Title: Earthquake prediction, a political problem?
Summary: Earthquakes are among natural hazards that threaten society. For this reason earthquake prediction is a field of research that arouses considerable nterest. An overview of the earthquake prediction problem is given. I show that the earthquake-prediction activities of scientists confront decision-makers with a fundamental trade-off between information and probability that the eartquake indeed occurs. However, this does not imply that scientists cannot contribute to alleviate the danger posed by earthquakes.
Roel Snieder holds the Keck Foundation Endowed Chair of Basic Exploration Science at the Colorado School of Mines. He received in 1984 a Masters degree in Geophysical Fluid Dynamics from Princeton University, and in 1987 a Ph.D. in seismology from Utrecht University. In 1993 he was appointed as professor of seismology at Utrecht University, where from 1997-2000 he was appointed as Dean of the Faculty of Earth Sciences. Roel served on the editorial boards of Geophysical Journal International, Inverse Problems, Reviews of Geophysics, the Journal of the Acoustical Society of America, and the European Journal of Physics. In 2000 he was elected as Fellow of the American Geophysical Union. He is author of the textbooks "A Guided Tour of Mathematical Methods for the Physical Sciences", "The Art of Being a Scientist", and "The Joy of Science" that are published by Cambridge University Press. Roel is a corresponding member of the Royal Netherlands Academy of Arts and Sciences. In 2011 he was elected as Honorary Member of the Society of Exploration Geophysicists, and in 2014 he received a research award from the Alexander von Humboldt Foundation. In 2016 Roel received the Beno Guterberg Medal from the European Geophysical Union and the Outstanding Educator Award from the Society of Exploration Geophysicists. From 2000-2014 he was a firefighter in Genesee Fire Rescue where he served for two years as Fire Chief.